sorting a List of Map String String

Question

I have a list variable created like this    List lt Map lt String  String gt  gt  list   new ArrayList lt Map lt String  String gt  gt      In my Android application  this list gets populated   just an example   Map lt String  String gt  map1   new HashMap lt String  String gt     map put  name    Josh         Map lt String  String gt  map2   new HashMap lt String  String gt     map put  name    Anna         Map lt String  String gt  map3   new HashMap lt String  String gt     map put  name    Bernie         list add map1   list add map2   list add map3     I am using list to show results in a ListView by extending BaseAdapter and implementing the various methods   My problem   I need to sort list in alphabetical order based on the map s key name  Question  What is a simple way to sort list in alphabetical order based on the map s key name   I can t seem to wrap my head around this   I have extracted each name from each Map into a String array  and sorted it Arrays sort strArray      But that doesn t preserve the other data in each Map  so i m not too sure how i can preserve the other mapped values

User · Answer

Test public void testSortedMaps         Map lt String  String gt  map1   new HashMap lt String  String gt         map1 put  name    Josh         Map lt String  String gt  map2   new HashMap lt String  String gt         map2 put  name    Anna         Map lt String  String gt  map3   new HashMap lt String  String gt         map3 put  name    Bernie         List lt Map lt String  String gt  gt  mapList   new ArrayList lt Map lt String  String gt  gt         mapList add map1       mapList add map2       mapList add map3        Collections sort mapList  new Comparator lt Map lt String  String gt  gt              public int compare final Map lt String  String gt  o1  final Map lt String  String gt  o2                return o1 get  name   compareTo o2 get  name                            Assert assertEquals  Anna   mapList get 0  get  name         Assert assertEquals  Bernie   mapList get 1  get  name         Assert assertEquals  Josh   mapList get 2  get  name

User · Answer

You should implement a Comparator lt Map lt String  String gt  gt  which basically extracts the  name  value from the two maps it s passed  and compares them   Then use Collections sort list  comparator    Are you sure a Map lt String  String gt  is really the best element type for your list though  Perhaps you should have another class which contains a Map lt String  String gt  but also has a getName   method

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try           java util Collections sort data                  new Comparator lt Map lt String  String gt  gt                          SimpleDateFormat sdf   new SimpleDateFormat                               MM dd yyyy                         public int compare final Map lt String  String gt  map1                              final Map lt String  String gt  map2                            Date date1   null  date2   null                          try                               date1   sdf parse map1 get  Date                                 date2   sdf parse map2 get  Date                               catch  ParseException e                                e printStackTrace                                                      if  date1 compareTo date2   gt  0                                return  1                            else if  date1 compareTo date2     0                                return 0                            else                               return -1                                                                             catch  Exception e

User · Answer

The following code works perfectly  public Comparator lt Map lt String  String gt  gt  mapComparator   new Comparator lt Map lt String  String gt  gt          public int compare Map lt String  String gt  m1  Map lt String  String gt  m2            return m1 get  name   compareTo m2 get  name              Collections sort list  mapComparator     But your maps should probably be instances of a specific class

User · Answer

There are many ways to solve the same  One of the easiest ways to solve using Java 8 is given below     As per your requirement  To sort in alphabetical order based on the map s key name  1st way     list   list stream               sorted  a b - gt   a get  name    compareTo b get  name                 collect Collectors toList       Or   list   list stream               sorted Comparator comparing map- gt map get  name                 collect Collectors toList       2nd way     Collections sort list  Comparator comparing map - gt  map get  name        3rd way    list sort Comparator comparing map- gt  map get  name

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Bit out of topic this is a little util for watching sharedpreferences based on upper answers may be for someone this will be helpful   SuppressWarnings  unused   public void printAll         Map lt String    gt  prefAll   PreferenceManager          getDefaultSharedPreferences context  getAll        if  prefAll    null            return            List lt Map Entry lt String    gt  gt  list   new ArrayList lt  gt         list addAll prefAll entrySet         Collections sort list  new Comparator lt Map Entry lt String    gt  gt              public int compare final Map Entry lt String    gt  entry1  final Map Entry lt String    gt  entry2                return entry1 getKey   compareTo entry2 getKey                           Timber i                                        Timber i  Printing all sharedPreferences        for Map Entry lt String    gt  entry   list            Timber i   s   s   entry getKey    entry getValue               Timber i

User · Answer

You need to create a comparator   I am not sure why each value needs its own map but here is what the comparator would look like   class ListMapComparator implements Comparator       public int compare Object obj1  Object obj2             Map lt String  String gt  test1    Map lt String  String gt   obj1           Map lt String  String gt  test2    Map lt String  String gt   obj2           return test1 get  name   compareTo test2 get  name               You can see it working with your above example with this   public class MapSort       public List lt Map lt String  String gt  gt  testMap              List lt Map lt String  String gt  gt  list   new ArrayList lt Map lt String  String gt  gt              Map lt String  String gt  myMap1   new HashMap lt String  String gt              myMap1 put  name    Josh             Map lt String  String gt  myMap2   new HashMap lt String  String gt              myMap2 put  name    Anna              Map lt String  String gt  myMap3   new HashMap lt String  String gt              myMap3 put  name    Bernie               list add myMap1            list add myMap2            list add myMap3             return list             public static void main String   args             MapSort ms   new MapSort             List lt Map lt String  String gt  gt  testMap   ms testMap             System out println  Before Sort      testMap            Collections sort testMap  new ListMapComparator              System out println  After Sort      testMap             You will have some type safe warnings because I did not worry about these   Hope that helps

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if you want to make use of lamdas and make it a bit easier to read     List lt Map lt String String gt  gt  results     Comparator lt Map lt String String gt  gt  sortByName   Comparator comparing x - gt  x get  Name        public void doSomething        results sort sortByName

User · Answer

Not really an answer to your question but I had a requirement to sort a List of maps by its values  properties  this will work in your case too  List lt Map lt String  Object gt  gt  sortedListOfMaps   someListOfMaps sorted Comparator comparing map - gt    String  map get  quot someKey quot      collect Collectors toList

[java] sorting a List of Map<String, String>

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